Generally, known spacecraft antenna tracking systems have been analog based. Such analog tracking systems typically consist of one or more arrays of feeds and a beam forming network (BFN) that are used in conjunction with a spacecraft reflector antenna system and connected to a modulator assembly (MA) and an analog tracking control receiver (TCR). Location of the elements in the feed array and the design of the BFN cause the reflector antenna system to produce a sum beam, and a null beam. For an incident beacon signal, the MA compares the phase and amplitude response of the sum beam to the phase and amplitude responses of the null beams and produces an amplitude modulated signal. The amplitude modulated signal is demodulated by the analog TCR and appropriate spacecraft control voltages are produced in response thereto.
The design and implementation of such analog systems can be problematic due to the fact that the location and number of elements in the feed array and the BFN are different for each reflector system. This makes the design of such elements very difficult when used in conjunction with shaped reflectors. In addition, the sum beam and nulls must be shaped such that the MA produces a linear response over the tracking range. In general, an analog tracking system only produces a linear response over a narrow angular region, which degrades spacecraft tracking performance and reduces spacecraft bias range. Further, the phase and amplitude between the sum beam and the nulls is critical and requires extensive testing to allow appropriate processing of the sum beam and null outputs by the MA.